1. Field of the Invention
Aspects of the present invention relate generally to an image forming apparatus and an auto color registration method thereof.
2. Description of the Related Art
An image forming apparatus can be a color laser printer, fax machine, photocopier, or the like. In an image forming apparatus a photosensitive medium is charged, a laser beam is scanned onto the photosensitive medium, to selectively expose the medium, and thereby form a latent electrostatic image. The latent electrostatic image is developed with toner of a predetermined color, using a developing device, to form a developed latent electrostatic image. The developed latent electrostatic image is transferred onto a printing medium, through pressurization and heating, thereby forming a printed image.
In general, yellow (Y), magenta (M), cyan (C) and black (B) colored toners are used in a developing device. Single color images, each comprising one of the four toners, are overlapped with one another, thereby forming a complete image. To provide a high quality image, the single color images are precisely overlapped with one another.
Errors can occur because of the misalignment of the single color images in a developing device. Errors can occur when processing using an optical lens, which can cause registration errors in a color image. For this reason, there is a need for an auto color registration operation that reduces such errors.
The occurrence of the auto color registration errors is an important factor, particularly in a single-pass color image forming apparatus having a plurality of color units arranged in series in a developing device. A tandem color laser printer 10, shown in FIG. 1, is a color image forming apparatus having such a structure.
When printing instructions are received by the color laser printer 10, a single color image is formed on each of photosensitive drums 12, 13, 14 and 15. After the images are formed, a pick-up roller 11 picks up paper S stacked in a paper feeding unit 16, to convey it toward a transfer unit 20. Paper, as referred to herein, includes any printable medium, for example, printer paper, fax paper, transparencies, and the like. The paper P passes between a paper transfer belt (PTB) 21, of the transfer unit 20, and the photo sensitive drums 12, 13, 14, and 15. The unit images for each color, are formed on the respective photosensitive drums 12, 13, 14, and 15 and are each transferred, in an overlapping fashion, onto the paper S, by corresponding transfer rolls 22, 23, 24, and 25. The paper S then passes through a fixing unit 30. The fixing unit 30 heats and presses the paper S, such that the images are fixed thereon. The paper S is then discharged from the color laser printer 10, via a paper discharging unit 40.
The printer 10 includes a feeding sensor 80 that senses the presence of the paper S that is fed through the printer 10. The printer 10 controls the timing and/or speed at which the paper S is fed to the photosensitive drums 12, 13, 14, and 15, and detects paper jams, using the feeding sensor 80.
The printer 10 includes a color registration sensor 60 disposed at one side of the PTB 21. As illustrated in FIG. 2, a color image pattern (test pattern) AcPt is printed at the PTB 21. The test pattern AcPt includes a plurality of color patterns Pt(Y), Pt(M), Pt(C), and Pt(B), respectively corresponding to the four colors of yellow (Y), magenta (M), cyan (C), and black (B).
The color registration sensor 60 receives light reflected from each of the color patterns, and then outputs a corresponding sensing signal. The color registration sensor 60 can recognize the positions P1 to P16, of the test pattern AcPt, using the structure of the test pattern. Through such pattern position recognition, the position of another color pattern is adjusted with respect to the position of any one color pattern, as a reference. Thus, the images for each color can be corrected to overlap with each other.
The aforementioned auto color registration method has a disadvantage in that while an image registration error for each color can be corrected, the method cannot be used to determine whether the position of a test pattern, partially printed on a paper, is correct. In a conventional method, because an image registration operation for each color is performed on the basis of an entire test pattern, it cannot be assured that a color image printed on a paper is properly positioned with respect to the paper. In general, an X offset in a horizontal direction and a Y offset in a paper transfer (vertical) direction are predetermined as reference information, allowing a color image to be positioned at the correct position on a paper. A color image is printed on a paper on the basis of the X and Y offsets. Since either of the X and Y offsets can be inappropriately set, or a scanning speed error of an LSU (Laser Scanning Unit), provided for each color to expose a respective photosensitive medium, occurs as the number of printing operations increases (although such X and Y offsets are appropriately set), a color image may not be printed at a correct position on a paper.
In a shuttle-type cartridge inkjet printer, auto color registration is performed by printing a test pattern for color registration, scanning the test pattern with a scanner, and then analyzing the scanned image. However, such scanning is inconvenient. A program tool for precisely analyzing a scanned image should be provided. Moreover, the auto color registration cannot be applied to color laser printers that do not have scanners.